Finding friction without coefficient?

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1. Aug 7, 2016

kelseybrahe

1. The problem statement, all variables and given/known data
We did a Prac where we attached ticker tape to a cart (905g), and the cart to a mass (249g) through a pulley. We then had to assess the tape and answer questions. We are asked to calculate the force acting upon the whole system and then calculate the force acting only upon the cart. We are not given the coefficient of friction.
I do not know the applied force (can I work this out?)

Average acceleration = 1.05m/s (initial velocity was zero/at rest?)

There is no inclination. The cart was on a flat, smooth desk.

2. Relevant equations
F=ma a(av)= v-u/t
Maybe Ek=1/2mv2

3. The attempt at a solution
Read somewhere that Ek=1/2mv2 can give you the coefficient of friction, but that didn't seem right at all. Other than that, completely lost.

2. Aug 7, 2016

haruspex

Define the system.
Net force, or just the force from the string tension?

3. Aug 7, 2016

kelseybrahe

I'm not entirely sure about either of these. The questions are very vague. I'm assuming by 'whole system' it means the combined mass of the cart and the mass pulling it. Which is 1154g.

For the second, the question simply states "it could be possible to use the same ticker tape to determine the friction acting on the cart only. How?"

4. Aug 7, 2016

haruspex

Ok. This is awkward, and I rather suspect the question is phrased wrongly. But let's take it at face value for now. What are the forces acting on the system (cart+string+weight).
Can you calculate what the acceleration would have been without friction?

5. Aug 7, 2016

kelseybrahe

Okay, thank you.
If it helps this is the first question:
"Calculate the difference in theoretical and measured acceleration, and then calculate The force of friction acting on the system."

Cart, string, pulley, weight and also the ticker time itself?

Yes! If I've done it correctly it's 0.47m/s^2.

Last edited: Aug 7, 2016
6. Aug 7, 2016

kelseybrahe

Sorry, part of my response has gone to the wrong place in there.

7. Aug 7, 2016

haruspex

do I have this right:
The first part asks for the frictional force acting on the system (as you have now defined it).
The second part does not ask you to calculate the frictional force acting on the cart only, but does ask how you could do that using the equipment.
?

You were advised to calculate the theoretical acceleration without friction. Do that.

8. Aug 7, 2016

kelseybrahe

Yes, I think so.
You've definitely got the first part right.
For the second (I have a cold and am a bit foggy - I'm sorry if I seem slow!) I think that it's asking for a formula or something. Do you know of one? If you don't, just go with equipment and see where that leads?

The theoretical acceleration is 0.47m/s^2

9. Aug 7, 2016

haruspex

10. Aug 8, 2016

kelseybrahe

m+M (mass total) divided by weight

(m= 249 (mass on string) M=905 (mass of cart) )

= 1.154/2.44
= 0.47

This was being told to assume the weight force was the only force acting on the system for that question.
Perhaps that can extend to the other questions as well.

Have I used the wrong formula?
I converted it to Kg, is that incorrect?

Last edited: Aug 8, 2016
11. Aug 8, 2016

haruspex

Small but important error. What is the relationship between force, mass and acceleration?

12. Aug 8, 2016

kelseybrahe

They are proportional to each other?
Increase force and mass, acceleration increases?

13. Aug 8, 2016

haruspex

Yes, but what is the equation?

14. Aug 8, 2016

kelseybrahe

Oh! F = ma

15. Aug 8, 2016

haruspex

Right. So is that what you wrote here?

16. Aug 8, 2016

kelseybrahe

No... Oh, okay.
So I would re-arrange it to be a=f/m?

17. Aug 8, 2016

haruspex

Yes.

18. Aug 8, 2016

kelseybrahe

Right. But how do I find f?
I have an answer for it but that is using the calculated acceleration of 1.05m/s^2. And that wouldn't be accurate, would it?

(Thank you for taking the time to go through this with me - I really appreciate it!)

19. Aug 8, 2016

haruspex

Let's get this step done first:

20. Aug 8, 2016

kelseybrahe

Oh sorry, I mean the f in the equation a=f/m
Because that's force, right? But the only way I can find force is by doing f=ma. I think?